Method for producing expandable polystyrene

ABSTRACT

A process for the preparation of expandable styrene polymers having a molecular weight M w  of greater than 170,000 g/mol, which comprises conveying a blowing agent-containing styrene polymer melt having a temperature of at least 120° C. through a die plate with holes whose diameter at the die exit is at most 1.5 mm, and subsequently granulating the extrudate, and expandable styrene polymers (EPS) having a molecular weight M w  of more than 170,000 g/mol with 0.05 to 1.5% by weight of internal water.

The present invention relates to a process for the preparation ofexpandable styrene polymers having a molecular weight Mw of greater than170,000 g/mol, in which a blowing agent-containing styrene polymer melthaving a temperature of at least 120° C. is conveyed through a die platehaving holes whose diameter at the die exit is at most 1.5 mm, andsubsequently granulating the extrudate, and to expandable styrenepolymers (EPS) having a molecular weight M_(w) of more than 170,000g/mol with 0.05 to 1.5% by weight of internal water.

Processes for the preparation of expandable styrene polymers, such asexpandable polystyrene (EPS), by suspension polymerization have beenknown for some time. These processes have the disadvantage that largeamounts of waste water are produced and have to be disposed of. Thepolymers have to be dried in order to remove internal water. Inaddition, the suspension polymerization generally results in broadbead-size distributions, which have to be sieved in a complex manner togive different bead fractions.

Furthermore, expanded (for example U.S. Pat. No. 3,817,669) andexpandable styrene polymers can be prepared by extrusion processes(GB-A-1,062,307). EP-A 668 139 describes a process for the economicalproduction of expandable polystyrene granules (EPS) in which the blowingagent-containing melt is produced by means of static mixing elements ina dispersion, hold and cooling step and is subsequently granulated.Owing to the cooling of the melt to a few degrees above thesolidification temperature, it is necessary to dissipate large amountsof heat.

WO 98/51735 describes expandable styrene polymers which comprisegraphite particles and have reduced thermal conductivity, these beingobtainable via suspension polymerization or via extrusion in atwin-screw extruder. The high shear forces in a twin-screw extrudergenerally result in significant reduction of molecular weight of thepolymer used, and/or some decomposition of additives such as flameretardants.

The cell number and foam structure obtained during foaming of theexpandable styrene polymers (EPS) are of decisive importance forachieving ideal insulation properties and good surfaces on the foams.EPS granules produced by extrusion can often not be foamed to give foamswith ideal foam structure.

It is an object of the present invention to remedy the above-mentioneddisadvantages and to provide an economical process for the production ofexpandable styrene polymer granules of small granule size and uniformgranule size distribution, which in particular can be foamed to givefoams with homogeneous structure and high cell number.

We have found that this object is achieved by the process described atthe outset and by expandable styrene polymers (EPS) having a molecularweight M_(w) of more than 170,000 g/mol which comprise 0.05 to 1.5% byweight of internal water.

It has been found that styrene polymers having molecular weights Mw ofless than 170,000 result in polymer abrasion during granulation. Theexpandable styrene polymer preferably has a molecular weight in therange from 190,000 to 400,000 g/mol, particularly preferably in therange from 220,000 to 300,000 g/mol. Owing to molecular weight reductioncaused by shearing and/or the effect of temperature, the molecularweight of the expandable polystyrene is generally about 10,000 g/molbelow the molecular weight of the polystyrene employed.

In order to obtain very small granule particles, the die swell after thedie exit should be as low as possible. It has been found that the dieswell can be influenced, inter alia, by the molecular weightdistribution of the styrene polymer. The expandable styrene polymershould therefore preferably have a molecular weight distribution with apolydispersity M_(w)/M_(n) of at most 3.5, particularly preferably inthe range from 1.5 to 2.8 and very particularly preferably in the rangefrom 1.8 to 2.6.

The styrene polymers employed are preferably transparent polystyrene(GPPS), high-impact polystyrene (HIPS), anionically polymerizedpolystyrene or high-impact polystyrene (A-IPS), styrene-α-methylstyrenecopolymers, acrylonitrile-butadiene styrene polymers (ABS),styrene-acrylonitrile (SAN), acrylonitrile-styrene acrylate (ASA),methacrylate-butadiene styrene (MBS), methylmethacrylate-acrylonitrile-butadiene-styrene (MABS) polymers or mixturesthereof or with polyphenylene ether (PPE).

To improve mechanical properties or thermal stability, the styrenepolymers mentioned may be blended, where appropriate with the use ofcompatibilizers, with thermoplastic polymers, such as polyamides (PAs),polyolefins, such as polypropylene (PP) or polyethylene (PE),polyacrylates, such as polymethyl methacrylate (PMMA), polycarbonate(PC), polyesters, such as polyethylene terephthalate (PET) orpolybutylene terephthalate (PBT), polyether sulfones (PES), polyetherketones or polyether sulfides (PES) or mixtures of these, generally intotal proportions up to a maximum of 30% by weight, preferably in therange from 1 to 10% by weight, based on the polymer melt. MIxtureswithin the ranges of amounts mentioned are also possible with, by way ofexample, hydrophobically modified or functionalized polymers oroligomers, rubbers, such as polyacrylates or polydienes, e.g.styrene-butadiene block copolymers, or biodegradable aliphatic oraliphatic/aromatic copolyesters.

Recycled polymers comprising the thermoplastic polymers mentioned, inparticular styrene polymers and expandable styrene polymers (EPS) mayalso be admixed with the styrene polymer melt in amounts which do notsubstantially impair its properties, generally in amounts of at most 30%by weight, in particular in amounts of from 1 to 10% by weight.

The blowing agent-containing styrene polymer melt generally comprisesone or more blowing agents in homogeneous distribution in a totalproportion of from 2 to 10% by weight, based on the blowingagent-containing styrene polymer melt. Suitable blowing agents are thephysical blowing agents usually employed in EPS, such as aliphatichydrocarbons having from 2 to 7 carbon atoms, alcohols, ketones, ethersor halogenated hydrocarbons. Preference is given to isobutane, n-butane,isopentane and n-pentane.

To improve foamability, finely dispersed droplets of internal water maybe introduced into the styrene polymer matrix. By way of example, thiscan be achieved via addition of water to the molten styrene polymermatrix. The addition preferably takes place prior to the feed of blowingagent. Kneading elements or static mixers can be used to achievehomogeneous distribution of the water.

The amount of water added is selected in such a way as to give theexpandable styrene polymers (EPS) an expansion capability a, defined asbulk density prior to foaming/bulk density after foaming, of at most125. A sufficient amount of water is generally from 0.05 to 1.5% byweight, based on the styrene polymer.

Expandable styrene polymers (EPS) with at least 90% of the internalwater in the form of droplets of internal water whose diameter is in therange from 0.5 to 15 μm form, on foaming, foams with adequate cellnumber and homogeneous foam structure.

The inventive expandable styrene polymer granules (EPS) generally have abulk density of at most 700 g/l.

The styrene polymer melt may furthermore comprise additives, nucleatingagents, plasticizers, flame retardants, soluble and insoluble inorganicand/or organic dyes and pigments, e.g. IR absorbers, such as carbonblack, graphite or aluminum powder, added together or at separatelocations. The amounts generally added of the dyes and pigments are inthe range from 0.01 to 30% by weight, preferably in the range from 1 to5% by weight. In order to obtain homogeneous and microdisperseddistribution of the pigments in the styrene polymer, it can beadvantageous, in particular in the case of polar pigments, to use adispersing agent, e.g. organosilanes or maleic-anhydride-grafted stryenepolymers, and to incorporate the material by mixing at low shear rates,e.g. below 30/sec, by way of an ancillary extruder or by way of a staticmixer. Preferred plasticizers are mineral oils, oligomeric styrenepolymers, and phthalates, in amounts of from 0.05 to 10% by weight,based on the styrene polymer.

The relatively high-molecular-weight styrene polymers enable the blowingagent-containing styrene polymer melt to be conveyed through the dieplate at a temperature in the range from 140 to 300° C., preferably inthe range from 160 to 240° C. Cooling into the region of the glasstransition temperature is not necessary.

The die plate is heated at least to the temperature of the blowingagent-containing polystyrene melt. The temperature of the die plate ispreferably in the range from 20 to 1001 C above the temperature of theblowing agent-containing polystyrene melt. Polymer deposits in the diesare thus prevented and salt-free granulation ensured.

In order to obtain marketable granule sizes, the diameter (D) of the dieholes at the die exit should be in the range from 0.2 to 1,5 mm,preferably in the range from 0.3 to 1.2 mm, particularly preferably inthe range from 0.3 to 0.8 mm. Even after die swell, granule sizes ofless than 2 mm, in particular in the range from 0.4 to 0.9 mm, can thusbe set specifically.

Besides the molecular weight distribution, the die swell can beinfluenced by the die geometry. Examples of suitable die geometries areshown in FIG. 1. The length (L) denotes the die zone whose diametercorresponds at most to the diameter (D) at the die exit. The die platepreferably has holes having an L/D ratio of at least 2. The L/D ratio ispreferably in the range from 3 to 10.

In general, the diameter (E) of the holes at the die entrance of the dieplate should be at least twice as large as the diameter (D) at the dieexit.

An embodiment of the die plate has holes having a conical inlet and aninlet angle α of less than 180°, preferably in the range from 30 to120°. In a further embodiment, the die plate has holes having a conicaloutlet and an outlet angle β of less than 90°, preferably in the rangefrom 15 to 45°. In order to produce specific granule-size distributionsof the styrene polymers, the die plate can be provided with holes ofdifferent outlet diameter (D). The various embodiments of the diegeometry can also be combined with one another.

A particularly preferred process for the preparation of expandablestyrene polymers having a molecular weight Mw of greater than 170,000g/mol comprises the following steps:

-   -   a) polymerization of styrene monomer and, if desired,        copolymerizable monomers,    -   b) degassing of the resultant styrene polymer melt,    -   c) mixing of the blowing agent and, if desired, additives into        the styrene polymer melt by means of static or dynamic mixers at        a temperature of at least 150° C.,    -   d) cooling of the blowing agent-containing styrene polymer melt        to a temperature of at least 120° C.,    -   e) discharge through a die plate having holes whose diameter at        the die exit is at most 1.5 mm, and    -   f) granulation of the blowing agent-containing melt.

In step f), the granulation can be carried out directly behind the dieplate under water at a pressure in the range from 1 to 10 bar.

The polymerization in stage a) and degassing in stage b) directlyprovides a polymer melt for impregnation by the blowing agent in stagec), and no melting of styrene polymers is needed. This is not only morecost-effective but also gives expandable styrene polymers (EPS) withlower styrene monomer contents, because the mechanical action of shearin the homogenizing section of an extruder—exposure which generallyleads to breakdown of polymers to give monomers—is avoided. In order tokeep the styrene monomer content low, in particular below 500 ppm, it isalso advantageous to minimize the amount of mechanical and thermalenergy introduced in all of the subsequent stages of the process.Particular preference is therefore given to the maintenance of shearrates below 30/sec and temperatures below 260° C., and also to shortresidence times in the range from 1 to 10 minutes, preferably from 2 to5 minutes, in stages c) to e). It is particularly preferable to useexclusively static mixers in the entire process. The polymer melt may betransported and discharged via pressure pumps, e.g. gear pumps.

Another method of reducing styrene monomer content and/or amount ofresidual solvent, such as ethylbenzene, consists in providing a highlevel of degassing in stage b) by means of entrainers, such as water,nitrogen, or carbon dioxide, or carrying out the polymerization stage a)by an anionic route. Anionic polymerization of styrene not only givesstyrene polymers with low stryene monomer content but also gives verylow styrene oligomer contents.

In order to improve the processing properties, the finished expandablestyrene polymer granules can be coated with glycerol esters, antistaticsor non-stick agents.

EXAMPLES

Unless otherwise stated in the examples, the examples were carried outusing a blowing agent-containing polystyrene melt comprising PS 158 Kfrom BASF Aktiengesellschaft having a viscosity number VN of 98 ml/g(M_(w)=280,000 g/mol, polydispersity Mw/Mn=3.0) and 6% by weight ofn-pentane.

Example 1

The blowing agent-containing polystyrene melt (6% by weight ofn-pentane) was conveyed at a throughput of 100 kg/h through a die platehaving 300 holes (diameter at the die exit (D) 0.4 mm, corresponding toshape A in FIG. 1). The melt temperature was 160° C. The expandablepolystyrene granules obtained had a uniform granule diameter of 1.0 mm.Styrene monomer content was determined as 400 ppm.

Increasing the melt temperature resulted in a reduction in the granulediameter. Melt temperature (° C.) Granule diameter (mm) 160 1.0 180 0.8200 0.65

Example 2

The blowing agent-containing polystyrene melt (6% by weight ofn-pentane) was conveyed at a throughput of 100 kg/h through a die platehaving 300 holes (diameter at the die exit (D) 0.4 mm, corresponding toshape A in FIG. 1). The temperature of the melt and the die plate wereeach 200° C. The expandable polystyrene granules obtained had a uniformgranule diameter of 0.65 mm. Melt temperature Die-plate (° C.)temperature (° C.) Granule diameter (mm) 200 180 0.80 200 200 0.65 200220 0.60 200 240 0.55

Example 3

The blowing agent-containing polystyrene melt (6% by weight ofn-pentane) was conveyed at a throughput of 100 kg/h through a die platehaving 300 holes (diameter at the die exit (D) 0.4 mm, with a conicalinlet angle corresponding to shape B in FIG. 1). The melt temperaturewas 180° C. Inlet angle (α) Granule diameter (mm) 180°  0.8 90° 0.7 45°0.65 30° 0.60

Example 4

The blowing agent-containing polystyrene melt (6% by weight ofn-pentane) was conveyed at a throughput of 100 kg/h through a die platehaving 150 holes (diameter at the die exit (D) 0.6 mm). The melttemperature was 180° C. Die shape according to FIG. 1 Granule diameter(mm) B 1.1 C 0.72

Example 5

The blowing agent-containing polystyrene melt (6% by weight ofn-pentane) was conveyed at a throughput of 100 kg/h through a die platehaving 150 holes (diameter at the die exit (D) 0.6 mm, corresponding toshape A). The melt temperature was 180° C. Additive Granule diameter(mm) None 1.1 3% by weight of mineral oil 0.8 5% by weight of butylbenzyl 0.8 phthalate 5% by weight of 0.75 low-molecular-weight GPPS(M_(w) = 5000)

Example 6

Polystyrenes having the properties from Example 1, but with differentpolydispersities Mw/Mn were employed. The blowing agent-containingpolystyrene melt (6% by weight of n-pentane) was conveyed at athroughput of 100 kg/h through a die plate having 300 holes (diameter atthe die exit (D) 0.4 mm, corresponding to shape A in FIG. 1). The melttemperature was 180° C. Mw/Mn Granule diameter (mm) 3 0.8 2 0.6 1.5 0.5

Example 7

0.1% by weight of water and 6% by weight of n-pentane were added to apolystyrene melt (PS 158 K) and the material was conveyed at athroughput of 100 kg/h through a die plate having 300 holes (diameter atthe die exit (D) 0.4 mm, corresponding to shape A in FIG. 1). The melttemperature was 160° C. The resultant expandable polystyrene granuleshad uniform granule diameter of 1.0 mm. The resultant granules wereexpanded in a current of steam to give foam particles with a fine-celledfoam structure which was evaluated under a microscope. The cell numberwas from about 4 to 4.5 cells/mm. More than 90% of the droplets ofinternal water had a diameter of 1.5 μm.

Example 8

Example 7 was repeated, except that 0.6% by weight of water was added tothe polymer melt. The resultant expandable polystyrene granules haduniform granule diameter of 1.0 mm. The resultant granules were expandedin a current of steam to give foam particles with a fine-celled foamstructure which was evaluated under a microscope. The cell number wasfrom about 8 to 8.5 cells/mm. More than 90% of the droplets of internalwater had a diameter of 10.5 μm.

Example 9

6% by weight of n-pentane, 0.3% by weight of a polystyrene-maleicanhydride graft copolymer as dispersing agent, and 0.8% by weight ofsilver metal pigment, based in each case on the polymer melt, were addedto a polystyrene melt (PS 158 K), and the material was conveyed at athroughput of 100 kg/h through a die plate having 300 holes (diameter atthe die exit (D) 0.4 mm, corresponding to shape A in FIG. 1). The melttemperature was 160° C. The resultant expandable polystyrene granuleshad uniform granule diameter of 1.0 mm. The resultant granules wereexpanded in a current of steam to give foam particles with a homogeneousfoam structure.

Example 10

Example 9 was repeated except that 0.8% by weight of a gold metal colorpigment and 0.3% by weight of organosilane as dispersing agent wereadded. The resultant expandable polystyrene granules had uniform granulediameter of 1.0 mm and were expanded with a current of steam to givefoam particles with homogenous foam structure.

Example 11

A polymer melt comprising blowing agent (polystyrene with a viscositynumber VN of 74 ml/g, average molecular weight M_(w) of 190,000 g/moland polydispersity M_(w)/M_(n) of 3.0, and 6% by weight of n-pentane)was conveyed at a throughput of 300 kg/h by way of a start-up valve withoil-heated, adjustable constrictor (start-up melt pressure about 180bar) through a die plate having 300 holes with 0.6 mm diameter at thedie exit. The resultant expandable polystyrene granules had a narrowparticle size distribution, 80% of the particles having a diameter inthe range from 0.62 to 0.8 mm. The residual monomer content wasdetermined as 325 ppm.

1. A process for the preparation of expandable styrene polymers having amolecular weight M_(w) of greater than 170,000 g/mol, which comprisesconveying a blowing agent-containing styrene polymer melt having atemperature in the range from 140 to 3005 C through a die plate withholes whose diameter at the die exit is at most 1.5 mm, and subsequentlygranulating the extrudate.
 2. A process as claimed in claim 1, whereinthe expandable styrene polymer has a molecular weight in the range from190,000 to 400,000 g/mol.
 3. A process as claimed in claim 1, whereinthe expandable styrene polymer has a molecular weight distributionhaving a polydispersity Mw/Mn of at most 3.5.
 4. A process as claimed inclaim 1, wherein the styrene polymer employed is transparent polystyrene(GPPS), high-impact polystyrene (HIPS), anacrylonitrile-butadiene-styrene polymer (ABS), styrene-acrylonitrile(SAN) or a mixture thereof or with polyphenylene ether (PPE).
 5. Aprocess as claimed in claim 1, wherein the blowing agent-containingstyrene polymer melt comprises, in homogeneous distribution, from 2 to10% by weight of one or more blowing agents selected from the groupconsisting of aliphatic hydrocarbons having from 2 to 7 carbon atoms,alcohols, ketones, ethers or halogenated hydrocarbons.
 6. A process asclaimed in claim 1, wherein the blowing agent-containing styrene polymermelt comprises plasticizers, such as mineral oils, oligomeric styrenepolymers and phthalates, in proportions in the range from 0.05 to 10% byweight, based on the styrene polymer.
 7. A process as claimed in claim1, wherein the blowing agent-containing styrene polymer melt is conveyedthrough the die plate at a temperature in the range from 160 to 2405 C.8. A process as claimed in claim 1, wherein the die plate is heated atleast to the temperature of the blowing agent-containing polystyrenemelt.
 9. A process as claimed in claim 1, wherein the diameter (D) ofthe die holes at the die exit is in the range from 0.2 to 1.2 mm.
 10. Aprocess as claimed in claim 1, wherein the die plate has holes having anL/D ratio (length (L) of the die zone, whose diameter corresponds atmost to the diameter at the die exit, to the diameter (D) at the dieexit) of at least
 2. 11. A process as claimed in claim 1, wherein thediameter (E) of the holes at the die entrance of the die plate is atleast twice as great as the diameter (D) at the die exit.
 12. A processas claimed in claim 1, wherein the die plate has holes having a conicalinlet with an inlet angle α of less than
 1805. 13. A process as claimedin claim 1, wherein the die plate has holes having a conical outlet withan outlet angle α of less than
 905. 14. A process as claimed in claim 1,wherein the die plate has holes having different exit diameters (D). 15.A process as claimed in claim 1, wherein the blowing agent-containingstyrene polymer melt comprises 0.05 to 1.5% by weight of water, based onthe styrene polymer.
 16. A process for the preparation of expandablestyrene polymers having a molecular weight Mw of greater than 170,000g/mol, comprising the following steps: a) polymerization of styrenemonomer and, if desired, copolymerizable monomers, b) degassing of theresultant styrene polymer melt, c) mixing of the blowing agent and, ifdesired, additives into the styrene polymer melt by means of static ordynamic mixers at a temperature of at least 1505 C, d) cooling of theblowing agent-containing styrene polymer melt to a temperature of atleast 1205 C, e) discharge through a die plate having holes whosediameter at the die exit is at most 1.5 mm, and f) granulation of theblowing agent-containing melt.
 17. A process as claimed in claim 15,wherein step f) is carried out directly behind the die plate under waterat a pressure in the range from 1 to 10 bar.
 18. An expandable styrenepolymer (EPS) obtainable by the process as claimed in claim 16, whichcomprises at most 500 ppm of styrene monomers.
 19. An expandable styrenepolymer (EPS) having a molecular weight M_(w) of more than 170,000g/mol, which comprises from 0.05 to 1.5% by weight of internal water.20. An expandable styrene polymer (EPS) as claimed in claim 19, whereinat least 90% of the internal water is present in the form of internalwater droplets having a diameter in the range from 0.5 to 15 μm.
 21. Anexpandable styrene polymer (EPS) as claimed in claim 19, whose expansioncapability α is at most
 125. 22. An expandable styrene polymer (EPS) asclaimed in claim 18, which takes the form of granules having a diameterof from 0.4 to 1.8 mm.
 23. An expandable granulated styrene polymer(EPS) as claimed in claim 22, which has a bulk density of at most 700g/l.
 24. An expandable styrene polymer (EPS) as claimed in claim 18,which comprises from 0.01 to 30% by weight of pigments.